Wärmebehandlung nach dem Schweißen

Post weld heat treatment (pwht) is an important step for relieving residual stresses in welded pressure equipment that could cause defects and hardening levels that exceed design specifications, as well as improving toughness and ductility to meet design criteria.

PWHT testing is required in most welding procedure qualification specifications such as EN 13445 and BS PD 5500, with time and temperature requirements detailed in tabular form in these documents.

Stressabbau

Post Weld Heat Treatment, commonly referred to as PWHT, can reduce internal stresses in steel components to prevent distortion and other issues that could lead to their failure. PWHT may also help mitigate hydrogen-induced cracking in some alloys as well as mitigate corrosion problems near welds in certain grades of metal.

Internal stress in steel may occur during forming, drawing and machining operations, leading to residual tensile or compressive stresses which lead to distortion and eventual service failure. PWHT offers an effective solution by heating the work piece above its lower critical temperature before gradually cooling it uniformly for stress relief.

Heating structures to this temperature decreases their yield strength and allows any residual welding stresses that exceed their new lower limit to exceed it, leading to plastic deformation which relieves residual stresses. This process, known as stress relief, involves soaking workpieces at this temperature for an adjustable period depending on size and complexity.

PWHT testing of larger components like air tanks and boilers to eliminate internal stresses before they can cause any lasting damage is often performed to ensure all internal stresses have been reduced to acceptable tolerance levels before final machining or finishing processes begin. It may also be done in fabrication stages in order to verify tolerance requirements prior to final finishing processes.

Hardness Control

Welding is an integral component of operating oil and gas pipelines, power plants and other industrial facilities. While welding provides numerous useful functions, its residual stresses can reduce material strength over time. To offset these stresses and ensure equipment safety for operation, heat treatment processes such as PWHT may be employed; PWHT reduces residual stresses while improving ductility and toughness to meet or surpass original design values.

PWHT is typically performed on carbon and alloy steels. The process involves heating them to an specified temperature before holding them there for an established amount of time – typically one hour per inch thickness. Temperature, cooling rate and holding time must all be carefully considered as improper procedures could cause temper embrittlement, distortion and reheat cracking issues.

PWHT can be an expensive process because it relies on costly energy sources to reach and sustain high temperatures necessary for effective stress relief. Furthermore, multiple cycles of PWHT over the lifetime of a component can increase overall energy consumption. Luckily, there are other alternatives that provide similar benefits – for instance composite materials can restore strength and stiffness without hot works or extensive welding processes being needed.

Strength Enhancement

Strength Enhancement is the ability to bolster one’s own physical strength; a subpower of Strength Manipulation and variation of Secondary Power.

Pwht methods (such as annealing, normalizing and tempering) involve heating and cooling of steel to remove or redistribute residual stresses. Other processes may involve precipitation, ageing or other metallurgical effects that improve mechanical properties of material such as hardness reduction, toughness increase or risk reduction of hydrogen cracking; specialist advice must be sought regarding times and temperatures necessary for these processes.

PWHT may also have an effect on the microstructure of weld metal, helping it become refined. This can improve fatigue performance of welds, leading to more favorable conditions being reached.

However, in many applications the decision to utilize PWHT is not determined solely by fatigue performance of the weld but instead determined by other factors, such as chemical and/or thickness requirements of specific codes or standards. Therefore, when making this decision it must be balanced against cost and potential adverse effects; pressure vessels and piping codes often mandate PWHT for weld metal over a certain thickness while in other instances susceptibility to stress corrosion cracking determines PWHT needs.

Ductility

Ductility refers to a material’s ability to deform plastically (that is, stretch and bend without breaking), as opposed to elastic deformation which can be reversed upon removal of stress. Most metals are relatively ductile; drawing wire or beating sheets is made easier thanks to metallic bonds allowing repeating atoms to slip past one another when stretched or shaped, while more fragile materials tend to crack at points where forces become concentrated.

Malleability is a material trait that measures how easily metal can be formed into thin sheets or other shapes, an essential quality in manufacturing applications involving small, thin components like those found in aerospace or automotive parts.

While ductility and malleability can be measured using various techniques, one of the best and most reliable indicators is through a standard tensile strength test. In this method, a flat or round specimen with grip sections on each end and thinner gauge sections in the middle (equivalent of dog bones or dumbbells) held tight by upper and lower jaws connected to a load cell is subjected to tension applied from an applied force source and stretched until breaking occurs, producing a stress-strain curve as the specimen stretched and then broke; the strain at point of necking serves as good indicator of material’s ductility although peak identification may prove challenging depending on sample formation quality.